This invention relates to a turn-off power semiconductor device in a semiconductor body having at least two contiguous zones of opposite conductivity type which in case of turnoff form a blocking p-n junction, and one zone of which has a constant dopant concentration (N.sub.1).
A turn-off power semiconductor component of this kind is described, e.g., in the IEEE, Transactions on Electronic Devices, Vol. ED-32, No. 9, 1985, p. 1830 ff. In FIG. 1 of this article, a typical doping profile of a gate turn-off (GTO) thyristor is illustrated. This GTO thyristor has a wide and uniformily weakly n-doped base zone, on one side of which also borders a narrow p-doped emitter zone provided with a very high doping gradient. Adjacent to the other side of the weakly n-doped base zone is a narrow p-doped base zone, also having a very high doping gradient, which is connected to a gate electrode. Contiguous to this heavily p-doped base zone is an emitter zone having a still higher n-doped concentration. The basic shape of the doping profile of such a GTO thyristor is represented in FIG. 1 of the present application.
Among conventional practices, e.g. from "Der Elektroniker", No. 11, 1985, p. 44 ff, there is the practice to connect in parallel with the GTO thyristor a so-called wiring capacitor. This wiring capacitor serves the purpose of limiting the positive rate of rise of off-state voltage upon turnoff of the GTO thyristor. Often this wiring capacitor has connected in series with it additionally a diode shunted by a parallel-connected resistor. Such an arrangement is generally known by the terms "RCD wiring" or "Snubber Circuit".
By applying a negative control pulse to the gate terminal of a conventional GTO thyristor, the latter can be extinguished, that is, turned off. In the turnoff process, after the end of the storage period, no further electrons are delivered by the n-emitter, so that in the semiconductor body a pure hole-current develops from the n-base zone via the p-base zone to the gate electrode. The space charge of these holes, which move approximately at saturation speed (v.sub.s =10.sup.7 cm/sec in silicon), is added to the space charge that exists through the doping of the semiconductor body. There then results in a shift in the location of the electric field peak from the doped p-n junction into the p-base zone, namely up to the point where the dopant concentration N.sub.A of the p-base zone corresponds exactly to the hole density. In addition, due to the alteration of the space charge there results in a contraction of the space-charge zone in the n-base zone or respectively an expansion of the space-charge zone in the p-base zone.
For the known doping profiles of the p-base zone with high doping gradients and the high current densities occurring in the practice (e.g. 100 A/cm.sup.2), this high doping gradient is so high that the expansion of the space-charge zone in the p-base zone cannot compensate for the contraction of the space-charge zone in the n-base zone. Accordingly, the voltage stability is reduced. Since during the fall period the current commutes upon turnoff of the GTO thyristor at up to several 1000 A/microsec into the RCD wiring, high voltage peaks caused by the parasitic RCD inductances will occur at still high current flow through the semiconductor body. Due to the low dynamic voltage stability, this may lead to destruction of the GTO thyristor.
The same problem occurs in principle in all semiconductor components including a p-n junction which is to be depleted quickly during current commutation, as for example in diodes or in bipolar transistors which are turned off by inversion of the base activation.